The present invention relates generally to an evaluating circuit for an inductive displacement pickup. Such evaluating circuits are used for example for evaluation of signals supplied from a differential transformer with a displaceable core and for outputting displacement pickup signals.
An evaluating circuit of the above mentioned type for any inductive displacement pickup is described for example in Valvo-Datenbuch, Professionelle Integrierte Analog- und Spezialschaltungen, Part 2, Dr. Alfred Huthig Verlag GmbH, Heidelberg, pages 818 and 819 under the title "Using the NE5521 Signal Conditioner in Mutli-Faceted Application". The above described circuit is briefly represented there by FIG. 5. It is described there with an application as a differential trafo displacement transmitter. Respective other applications with inductive displacement pickups are also correspondingly possible, as contained in the above mentioned article. The same is true for the present invention. In order to be brief however only those examples which are used as differential trafo displacement pickups are described.
The known circuit shown in FIG. 5 has a sinus generator which is controlled by a driver for supplying a primary winding 10 of a differential trafo displacement pickup. At the secondary side two secondary windings 11.1 and 11.2 are provided, and their output voltages change oppositely when a displaceable core 12 is displaced. The driver circuit operates for doubled voltage stroke. The secondary-side difference signal is rectified by a phase-coupled rectifier with consideration of the phase of the driver signal. This rectified signal has a relatively high waviness ( standing wave ratio) and therefore it must be positively filtered so as to finally form a useable displacement pickup-direct voltage UW after reinforcing of the filtered voltage.
For providing an output signal which is as smooth as possible, other evaluating circuits are also known. In a circuit described in the European Patent document EP 88 903 834, a balancing process (inductive variation) is performed by means of a balancing direct voltage. The balancing value of this direct voltage is directly proportional to the displacement detected by the displacement pickup. It is obvious that this voltage can be obtained without any waviness. However, the circuitry expenses are considerable.